The invention is directed to the field of electronic reproduction technology and relates to a method and to an apparatus for the calibration of color values in color conversion in apparatus and systems for image processing.
Reproduction technology is concerned with methods for reproducing image originals in printing, wherein a master is prepared from an original as the basis for a printing form. The reproduction of the original occurs in a printing press with the printing form.
The process for producing a master is generally composed of the steps of image input, image processing, and image output.
In the image input with, for example, a color image scanner, three primary color value signals (R, G, B) are acquired by trichromatic and pixel-by-pixel or line-by-line, optoelectronic scanning of the color originals to be reproduced, whereby the individual color value triads represent the color components "red" (R), "green" (G) and "blue" (B) of the pixels scanned in the color original. The analog color value signals are converted into digital color values and are stored for the subsequent image processing.
In the image processing, the color values (R, G, B) are usually first converted into color separation values (C, M, Y, K) with a basic color correction according to the rules of subtractive color mixing. These color separation values are a measure for the dosing of the inks "cyan" (C), "magenta" (M), "Yellow" (Y and "black" (K) or, respectively, for the sizes of the raster points or raster percentages employed in the later printing process. Over and above this, further color corrections are implemented in the image processing with the object of improving the image reproduction, of compensating deficiencies, or of undertaking editorial changes.
Following the image processing, the image output occurs with a unit suitable for this purpose, for example with a color separation recorder for the raster recording of the color separations on a recording material (film). The methods currently standard in the reproduction of color originals are essentially based on the principle of color density measurement with a direct separation of the color values (R, G, B) acquired in the color image scanner into the color separation values (C, M, Y, K).
The separation occurs according to the techniques of traditional photographic color separation production on the basis of color masking. These separations are specially matched to the type of color image scanner apparatus employed with the spectral sensitivity and signal pre-distortion thereof.
The corrections of the color value signals (R, G, B) necessary for the printing process are experimentally identified in practice by visual comparison between color original and the printing result. An adaptation of the corrections to different printing processes as well as to different color scanning apparatus having different spectral sensitivity is often difficult and time-consuming.
Since the demand is frequently currently raised that different color image scanners be connected to one color image processing system, or that one color image scanner be connected to different color image processing systems, the introduction of a defined standard at the interface between color image scanner and color image processing is of great advantage. For that purpose, the color value signals produced in the color scanner must be calibrated onto the respective, internal standard of the color image processing system.
It is already known from the literature, for example from the periodical "Der Druckspiegel", No. 6, 1991, pages 580-592, "Farbe in der Bildverarbeitung", to transform the color values of an apparatus-dependent input color space acquired in a color image scanner for a universal color presentation into color values of an apparatus-independent communication color space on the basis of color space transformation, to undertake the corresponding corrections with reference to the transformed color values, and to then convert the corrected color values into the corresponding process color values of an apparatus-dependent output color space on the basis of a further color transformation.
The known methods for color space transformation are based on colorimetricelly defined values and can therefore not be employed in color image scanners having undefined color filters, i.e. having color filters that are not matched to the standard spectral value curves of the CIE (Commission Internationale L'Eclairage).